xfs: cleanup __FUNCTION__ usage

__FUNCTION__ exists only for backwards compatibility reasons
with old gcc versions. Replace it with __func__.

Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>